1. Technical Field
The invention relates generally to axle suspensions for wheeled vehicles. More particularly, the invention relates to lift axle suspension systems. Specifically, the invention relates to lift axle suspension systems for selectively lowering and raising the wheels of a vehicle into and out of engagement with the road surface.
2. Background Information
With the advent following World war II of large load carrying capacity trucks and trailers in this country, came the need to provide add-on axles, for increasing the capacity of trucks over that of the chassis-cab design which was manufactured with a limited number of axles. While add-on axles effectively increased carrying capacity, it was soon realized that as the number of load bearing axles increased on a given vehicle, a number of difficulties arose. Specifically, tire scuffing, loss in fuel economy and the inability to safely corner, all were problems associated with add-on type axles. Mitigation of these problems was of primary concern to the industry, which concern resulted in the development of the lift axle suspension system. Such a suspension system could be selectively raised from the road surface or lowered into engagement with the road surface when needed, thereby mitigating the aforementioned problems.
Many types and varieties of lift axle suspensions have been designed and employed since their inception with varying degrees of success or failure. Among the most successful are those of the parallelogram air-powered lift type produced for example by Suspensions Inc., of Canal Fulton, Ohio, an example of which is sold as model number LP-K13.75-12-002. Another type is the upwardly biased leaf spring produced by Turner Quick-Lift Corporation of Canton, Ohio, such as disclosed in U.S. Pat. No. 3,185,621. Both of these types of units have the advantage of being safe, durable, effective for their intended purpose, useful in both the pusher (forward of the drive axle), and tag (rearward of the drive axle) positions and on both truck chassis and trailers alike.
While these two types of lift axle suspensions have operated effectively and solved an important need in the art, each design presents a number of drawbacks. Specifically, in the parallelogram type air-lift suspension, the length of the overall unit prohibits its use where only short frame lengths are available to accommodate the suspension. While an upwardly biased leaf spring lift axle that is capable of fitting onto relatively short places has been developed, as disclosed in U.S. Pat. No. 3,904,220, it has not always achieved optimized lift height and spring life, and for certain purposes the overall length of the unit is too large.
Further, the parallelogram air-lift axle has presented a number of other drawbacks. These lift axle suspensions have not fully utilized the lift capacity of the air operated lift bellows, or air spring. The lift air spring in the prior suspensions generally fan open, such that one side of the spring remains relatively compressed while the opposing side is fully extended. When the lift air spring is fanned open in this manner, the spring's effective operating length is significantly less than the available operating length of the air spring.
In order to increase the overall effective operating length of the lift air spring, the rigid connection between the air spring plate and the beam, as embodied in model number MTL-50 sold by Turner Suspensions of Canton, Ohio, was changed to a pivoting connection which was offset from the centerline of the plate as included in Suspensions, Inc. model number LP-002. In this manner, the effective operating length of the air spring was increased. This offset pivot point made the expansion and contraction of the air spring less controllable, and therefore a large area below the lift spring was included in the suspension to assure that the spring would not be damaged during operation. This increased area under the lift spring significantly increases space requirements for the suspension system.
Another problem associated with previously existing lift axles is that they provide reduced lateral control for the associated axle and wheels and as such, tire scuffing is a consistent problem when the tires carried by the lift axle engage the road surface.
The parallelogram air-lift axle also includes an inherent interference between the torque rod and the suspension air bellows, such that as the air spring deflects during normal operation, the torque rod connection can contact the bellows and damage or destroy the air spring.
While some known lift suspensions are steerable in that the suspension includes caster bearings which allow the tire-wheel assembly to rotate relative to the axle in response to forces acting on the tire-wheel assembly, these systems only provide a limited angle through which the caster may rotate. The angle of steer is generally limited by the positioning and location of the hanger bracket.
Therefore, the need exists for a lift axle suspension system wherein the entire capacity of the lift air spring is utilized as the associated plates are substantially parallel when the air spring is in both the fully extended and fully compressed positions. Moreover, a need exists for such a lift axle suspension system where the expansion and compression of the lift air spring is controlled such that the spring's position during the bellow's expansion and contraction is controlled and predefined. Still further, the need exists for such a lift axle suspension system which will fit in a small space beneath a vehicle, is lightweight, and in which the suspension air spring does not interfere with the components of the suspension assembly.